1. Field of the Invention
The present invention relates to a method for improving the optical separation of needle-shaped fluorescent layers which are formed by vapor deposition onto a substrate, in particular for an X-ray detector.
2. Description of the Prior Art
Needle-shaped fluorescent layers can be produced by the vapor deposition of alkali metal halides (for example NaCl, CsI, CsBr, RbBr, RbFBr, RbFCl) as absorber material for X-ray radiation or as host lattice for the dopants such as, for example, NaI, EuBR2, TII, GaBr, EuCl2 etc. (Stevels, Schrama; xe2x80x9cVapour-Deposited CsI:Na Layers, I. Morphologic and Crystallographic Propertiesxe2x80x9d, Philips Res. Repts 29, 340-352, 1974). The vapor deposition of fluorescent materials is an alternative for the production of binderless fluorescent storage panels (European Application 0 175 578). In order to obtain a good modulation transfer function (MTF) for these fluorescent layers, there is a need for small needle diameters ( less than 40 xcexcm). The needles must be surrounded with cracks (gaps) of 0.3 to 3 xcexcm for the purpose of optical separation (Eckenbach, xe2x80x9cComputersimulation des Photonentransports in CsI-Eingangsschirmen von Rxc3x6ntgenbildverstxc3xa4rkernxe2x80x9d [xe2x80x9cComputer simulation of the photon transport in CsI input screens of X-ray image intensifiersxe2x80x9d], Unsere Forschung in Deutschland, Volume IV, 51-53, Philips, 1989). However, this requirement can be met only with very special environmental conditions during the production of the fluorescent layers.
The utilization of the differences in expansion between the substrate and the vapor-deposited fluorescent layer has become established as the most frequently practiced method for producing the required cracks between the needles. In this case, vapor deposition has been performed at a relatively high substrate temperature, and upon coolingxe2x80x94given a correct choice of materialxe2x80x94the fluorescent layer shrinks more than the substrate situated therebelow, and this leads to thermal stresses and therefore to cracks or gaps (Stevels, Schrama; xe2x80x9cVapour-Deposited CsI:Na Layers, II. Screens for Application in X-ray Imaging Devicesxe2x80x9d, Philips Res. Repts 29, 353-362, 1974). Examples of the correct choice of material are CsI on aluminum and CsBr on glass. This method has the disadvantage that the needle diameter of the fluorescent layer increases substantially with increasing substrate temperature, and therefore the MTF decreases (Waterbeemd, Oosterhout; xe2x80x9cEffect of the Mobility of Metal Atoms on the Structure of Thin Films Deposited at Oblique Incidencexe2x80x9d, Philips Res. Repts 22, 375-387; 1967).
A second variant for producing fissures or cracks in the fluorescent layer which are necessary for a good MTF is to carry out vapor deposition at low substrate temperatures (50xc2x0 C.), and to subject the layer thereafter to heat treatment (500xc2x0 C.) (Stevels, Schrama; xe2x80x9cVapour-Deposited CsI:Na Layers, I. Morphologic and Crystallographic Propertiesxe2x80x9d, Philips Res. Repts 29, 340-352, 1974). The disadvantage of this method is grounded in the re-crystallization of the fluorescent layer at the tempering step. A xe2x80x9cgrain coarseningxe2x80x9d occurs in this case, thereby leading to larger block widths and thus to a poorer MTF.
A third variant is xe2x80x9coblique vapor depositionxe2x80x9d. In this method, the vapor jet of the fluorescent material strikes the substrate at a flat angle during vapor deposition. A good separation between the needles is achieved by xe2x80x9cshadowingxe2x80x9d as early as in the seeding phase (European Application 0 175 578). In the practical implementation, either the substrate is arranged rotating obliquely relative to the evaporator vessel, or the vapor jet is xe2x80x9cdirectedxe2x80x9d to a horizontally rotating substrate in a fashion channeled by xe2x80x9cchimneysxe2x80x9d. Disadvantages of this technique are to be seen in that, firstly, the vapor deposition of large substrates (Ø greater than 40 cm) is very expensive in terms of apparatus and, secondly, the needles grow obliquely on the substrate in accordance with the angle of incidence of the vapor jet (Eckenback; Philips, xe2x80x9cComputersimulation des Photonentransports in CsI-Eingangsschirmen von Rxc3x6ntgenbildverstxc3xa4rkernxe2x80x9d [xe2x80x9cComputer simulation of the photon transport in CsI input screens of X-ray image intensifiersxe2x80x9d], Volume IV, 51-53, 1989). This results in different layer properties over the surface (vignetting effect in the case of X-ray excitation).
A fourth variant of the known method of production is vapor deposition of the fluorescent material onto a structured support (wire fabric, photoetched substrate etc.), the xe2x80x9cdepressionsxe2x80x9d in the substrate constituting a barrier against surface diffusion, and the xe2x80x9craisedxe2x80x9d surface elements exerting a xe2x80x9cshadow effectxe2x80x9d on the xe2x80x9cneighborsxe2x80x9d (Stevels, Schrama; xe2x80x9cVapour-Deposited CsI:Na Layers, II. Screens for Application in X-ray Imaging Devicesxe2x80x9d Philips Res. Repts 29, 353-362, 1974).
In addition to the frequently very complicated steps in the production of the fluorescent layers according to one of these known methods, they do not all lead to an optimum optical separation of the needle-shaped fluorescent layers.
It is known from German OS 198 52 326, German OS 195 16 450, German OS 42 19 347 and U.S. Pat. No. 3,825,763 for fluorescent layers to be vapor deposited with substrate temperatures of 50xc2x0 to 250xc2x0 C.
Moreover, in connection with the production of oxide layers it is known from European Application 0 698 932 to feed cooled reactive gas in order to improve the crystallinity of the layer produced.
It is an object of the present invention to provide a method of the type described above so that, upon vapor deposition, needle structures are produced which are of the smallest possible diameter and are optically separated from one another effectively by cracks, thus producing an improvement in the optical separation of the fluorescent layers.
This object is achieved in a method according to the invention wherein vapor deposition is controlled so that a fluorescent layer is deposited on a substrate with a density which is reduced in comparison with to density that the fluorescent material has as a solid, the density, preferably reduced by 5% to 50%, in the deposited layer producing fissures between the fluorescent needles and pores or defects and dislocations in the crystal structure (lattice defects).
In order to reduce the density, in an embodiment of the invention, before striking the substrate the vapor jet is cooled, preferably by conducting cool inert gas, for example argon, through the vapor-deposition apparatus.
In a further version of this embodiment of the invention the gas pressure of the inert gas, which is introduced by a control valve into the vapor-deposition apparatus and discharged again via a pump, is below 10 Pa, preferably between 1 Pa and 3 Pa.
A very considerable reduction in density results from this use of cooled inert gas. The temperature of the inert gas is, for example, between 0xc2x0 C. and 100xc2x0 C., preferably at approximately room temperature, as a result of which the vapor, at a temperature of approximately 650xc2x0 C. can, of course, be strongly cooled, when the vapor strikes substrate, which also is preferably cooled so that its temperature is preferably kept at between 50xc2x0 C. and 200xc2x0 C. A substantial reduction in density, and thus a good optical separation of the fluorescent needles which results therefrom, are ensured by the combination of the cooling of the vapor jet and the cooling of the substrate to a temperature which is much lower than the vapor temperature.
Physically, the reduced density can be explained with larger lattice spacings in the case of phase conversions of polymorphous crystals (NaCl type or CsCl type in the case, for example, of CsBr, CsCl, TlBr etc.) and/or by the formation of lattice defects by means of a high vapor deposition rate. In an embodiment of the invention the vapor deposition rate is preferably above 1 mg cmxe2x88x922minxe2x88x921. The reduced density also can be explained by the xe2x80x9cfreezingxe2x80x9d of the lower density of the liquid. The density of CsBr, for example, is 3.05 g cmxe2x88x923 in the liquid state, and 4.44 g cmxe2x88x922 in the solid state. The ratio at which the reduced density becomes noticeable either as fissures between needles or as lattice defects inside the needles can be influenced by the vapor deposition rate.
In the event of an excessively high inert gas pressure and/or excessively low substrate temperature, however, no more fluorescent needles can be formed because new seeds are always forming on the surface, which can no longer be connected to the already condensed layer. Substrate temperatures of between 50xc2x0 C. and 200xc2x0 C. and argon pressures of between 1 Pa and 3 Pa, for which a 10-30% reduction in density is achieved, have proved to be virtually optimum.